Trachea, identified by the presence of hyaline cartilage in its wall. To the left of the cartilage is the mucosa, including epithelium and its underlying c.t.. The cartilage ring is immediately covered, on both surfaces, with bright pink perichondrium.

Trachea at higher magnification, showing pseudostratified ciliated columnar epithelium lining the lumen. A wide blood vessel lies in the lamina propria below. Hyaline cartilage is at the bottom of the picture.

Detail of pseudostratified ciliated columnar epithelium lining the trachea. Note the presence of pale goblet cells. Foreign particles are trapped in mucus secreted by goblet cells and then moved up (and out of) the respiratory tract by the beating cilia. A pale and quite thick basement membrane underlies this epithelium.

Posterior wall of the trachea with smooth muscle (pink strand near epithelium) replacing cartilage in the wall. The cartilage is basophilic in color. Structurally, it is a C-shaped ring, incomplete posteriorly, but with the trachealis muscle spanning the gap between the ends of the "C".

The trachealis muscle (smooth muscle) is the bright pink band in the middle of the field. Notice how cellular the c.t. under the epithelial layer is. Wandering blood cells, especially lymphocytes and eosinophils, are usually found here.

Longitudinal, parasagittal section of trachea showing a series of darkly stained cartilage rings cut across. The rings are thicker anteriorly (at the right) than they are posteriorly (at the left). If this were a true mid-sagittal section, the cuts at the left would be of trachealis muscle instead of cartilage, so this section is clearly off-cen
ter. The lining of the wide open lumen would be typical, pseudostratified "respiratory" epithelium.
NOTE: Primary bronchi are essentially like the trachea in structure, and we have no example to show you. Lower portions of the respiratory tract appear in sections of lung and they show progressive loss of the various components characteristic of the trachea; that is, less and less cartilage, progressively lower epithelium, gradual loss of gob
let cells, and finally loss of cilia and smooth muscle.

An intrapulmonary bronchus with several separate pieces of hyaline cartilage in its wall. A thin layer of bright pink smooth muscle lies between the cartilage and the mucosa. (The mucosa, as always, consists of epithelium and underlying connective tissue.) In the upper left quadrant of the field is a large branch of the pulmonary artery, with it
s bright pink tunica media (smooth muscle coat). Arterial branches accompany the branching respiratory tree all along the way.

Detail of previous wall, showing the large chondrocytes of the cartilage. The epithelium lining the lumen looks pseudostratified still. A layer of pink smooth muscle lies between it and the cartilage. Considerable elastic tissue lies in the respiratory wall as well, but is not distinguishable in H&E stain.

A branching portion of the respiratory tree. No cartilage is visible here, so we'll say it's a bronchiole. Note a lymphatic nodule in the fork of the tree; such nodules may be found randomly along the respiratory tree. Note also the cross-cuts of pulmonary artery branches accompanying the tree. Cuts of alveoli of the lung fill the surrounding
field.

b = respiratory bronchiole with alveolus (a) in its wall. Most of the wall of the bronchiole has a definite line of dark along it, signifying a cuboidal or columnar epithelium (simple, rather than pseudostratified by now).

d & c = alveolar duct. Its wall consists almost entirely of alveoli, which have only a simple squamous lining, too flat to be visible here.

Another view of terminal branches of respiratory tree in the lung. In the middle of the field is a small terminal bronchiole branching off into three or four alveolar ducts to the right. The bronchiole has a definite, pink epithelial lining, while the walls of the alveolar ducts consist mainly of alveoli. The bronchiole is called terminal simpl
y because it's the last generation of bronchiole before alveoli start to appear in the wall. Just before the alveolar ducts branch off, you can see a couple of small alveolar outpocketings in the bronchiole wall, thus making this short segment a respiratory bronchiole. (Note: along the top of this field is a wall of a bronchus, with two very sma
ll, basophilic patches of cartilage just under the pink layer of smooth muscle.)

The outer reaches of two lung lobules, with a connective tissue septum running vertically between them. The lower edge of the tissue here is the visceral pleura. Lymphatic vessels (a) and veins (b) run in the septa at the periphery of each lobule. Arteries, as we've noted previously, typically follow the branchings of the respiratory tree itsel
f. At (c) there is a bronchiole. The mucosa lining its lumen is typically thrown into folds or scallops because of contraction of smooth muscle and elastic fibers in the wall.

Detail of inter-alveolar septa which form the shared walls of alveoli. The large spaces here are alveoli, filled with air. In the septa, orange is the red blood cells and brown is the nuclei of the capillary endothelium and alveolar cells. Every surface facing the alveolar air spaces is lined by simple squamous epithelium of the alveolar walls.

Detail of the septa between alveolar spaces. The small spaces within the septa are empty capillary lumens. Nuclei belong mainly to endothelial cells and alveolar epithelium; it would take EM to identify which is which with certainty. A few might also be fibroblasts or wandering c.t. cells. Very fine collagenous and elastic fibers also accompan
y the capillaries. As you can see, especially in the capillaries in the center and in the upper left of the field, the combined thickness of capillary endothelium plus simple squamous alveolar epithelium separating blood from air, is extremely thin.

EM showing basal lamina (1) between squamous alveolar epithelium
(2 = Type I cell) and capillary endothelium (3). The nucleus at upper right belongs to the
endothelial cell lining the capillary. The dark structure is a red blood cell.
The capillary plus the alveolar linings on both sides constitute the interalveolar septum
that lies between two alveolar spaces.

EM of alveolar wall with a Type II cell (or granular pneumocyte). It is a rounded or cuboidal cell,
in contrast to the very flat Type I alveolar lining cell. To compare the differences in cell
thickness, look in the lower part of the micrograph where you see capillary endothelium (1),
basal lamina (2), and alveolar Type I epithelium (3) making up the very thin blood-air interface.
The large,
vacuolated, rather ragged looking vesicles in the cytoplasm of the Type II cell are lamellar
bodies containing the precursor of alveolar surfactant.